Since glibc 2.19:
_DEFAULT_SOURCE
In glibc up to and including 2.19:
_BSD_SOURCE || _BSD_SOURCE

DESCRIPTION

inet_aton()
converts the Internet host address cp from the
IPv4 numbers-and-dots notation into binary form (in network byte order)
and stores it in the structure that inp points to.
inet_aton()
returns nonzero if the address is valid, zero if not.
The address supplied in
cp
can have one of the following forms:

a.b.c.d

Each of the four numeric parts specifies a byte of the address;
the bytes are assigned in left-to-right order to produce the binary address.

a.b.c

Parts
a
and
b
specify the first two bytes of the binary address.
Part
c
is interpreted as a 16-bit value that defines the rightmost two bytes
of the binary address.
This notation is suitable for specifying (outmoded) Class B
network addresses.

a.b

Part
a
specifies the first byte of the binary address.
Part
b
is interpreted as a 24-bit value that defines the rightmost three bytes
of the binary address.
This notation is suitable for specifying (outmoded) Class A
network addresses.

a

The value
a
is interpreted as a 32-bit value that is stored directly
into the binary address without any byte rearrangement.

In all of the above forms,
components of the dotted address can be specified in decimal,
octal (with a leading
0),
or hexadecimal, with a leading
0X).
Addresses in any of these forms are collectively termed
IPV4 numbers-and-dots notation.
The form that uses exactly four decimal numbers is referred to as
IPv4 dotted-decimal notation
(or sometimes:
IPv4 dotted-quad notation).

inet_aton()
returns 1 if the supplied string was successfully interpreted,
or 0 if the string is invalid
(errno
is
not
set on error).

The
inet_addr()
function converts the Internet host address
cp from IPv4 numbers-and-dots notation into binary data in network
byte order.
If the input is invalid,
INADDR_NONE
(usually -1) is returned.
Use of this function is problematic because -1 is a valid address
(255.255.255.255).
Avoid its use in favor of
inet_aton(),
inet_pton(3),
or
getaddrinfo(3),
which provide a cleaner way to indicate error return.

The
inet_network()
function converts
cp,
a string in IPv4 numbers-and-dots notation,
into a number in host byte order suitable for use as an
Internet network address.
On success, the converted address is returned.
If the input is invalid, -1 is returned.

The
inet_ntoa()
function converts the Internet host address
in, given in network byte order, to a string in IPv4
dotted-decimal notation.
The string is returned in a statically
allocated buffer, which subsequent calls will overwrite.

The
inet_lnaof()
function returns the local network address part
of the Internet address in.
The returned value is in host byte order.

The
inet_netof()
function returns the network number part of
the Internet address in.
The returned value is in host byte order.

The
inet_makeaddr()
function is the converse of
inet_netof()
and
inet_lnaof().
It returns an Internet host address in network byte order,
created by combining the network number net
with the local address host, both in
host byte order.

The structure in_addr as used in
inet_ntoa(),
inet_makeaddr(),
inet_lnaof()
and
inet_netof()
is defined in
<netinet/in.h>
as:

typedef uint32_t in_addr_t;

struct in_addr {
in_addr_t s_addr;
};

ATTRIBUTES

For an explanation of the terms used in this section, see
attributes(7).

Interface

Attribute

Value

inet_aton(),
inet_addr(),
inet_network(),
inet_ntoa()

Thread safety

MT-Safe locale

inet_makeaddr(),
inet_lnaof(),
inet_netof()

Thread safety

MT-Safe

CONFORMING TO

inet_addr(),
inet_ntoa():
POSIX.1-2001, POSIX.1-2008, 4.3BSD.

inet_aton()
is not specified in POSIX.1, but is available on most systems.

NOTES

On x86 architectures, the host byte order is Least Significant Byte
first (little endian), whereas the network byte order, as used on the
Internet, is Most Significant Byte first (big endian).

inet_lnaof(),
inet_netof(),
and
inet_makeaddr()
are legacy functions that assume they are dealing with
classful network addresses.
Classful networking divides IPv4 network addresses into host and network
components at byte boundaries, as follows:

Class A

This address type is indicated by the value 0 in the
most significant bit of the (network byte ordered) address.
The network address is contained in the most significant byte,
and the host address occupies the remaining three bytes.

Class B

This address type is indicated by the binary value 10 in the
most significant two bits of the address.
The network address is contained in the two most significant bytes,
and the host address occupies the remaining two bytes.

Class C

This address type is indicated by the binary value 110 in the
most significant three bits of the address.
The network address is contained in the three most significant bytes,
and the host address occupies the remaining byte.

Classful network addresses are now obsolete,
having been superseded by Classless Inter-Domain Routing (CIDR),
which divides addresses into network and host components at
arbitrary bit (rather than byte) boundaries.

EXAMPLE

An example of the use of
inet_aton()
and
inet_ntoa()
is shown below.
Here are some example runs:

$ ./a.out 226.000.000.037 # Last byte is in octal
226.0.0.31
$ ./a.out 0x7f.1 # First byte is in hex
127.0.0.1

SEE ALSO

COLOPHON

This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.